nios2: Add -mcustom-fpu-cfg=fph2

nios2: Add -mcustom-fpu-cfg=fph2

The new -mcustom-fpu-cfg=fph2 option variant is useful to build a
multilib for the "Nios II Floating Point Hardware 2 Component":

https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/ug/ug_nios2_custom_instruction.pdf

Directly using the corresponding -mcustom-insn=N options for this
floating-point unit leads to a combinatorial explosion in the potential
count of multilibs which may break the build.

gcc/

	* config/nios2/nios2.c (NIOS2_FPU_CONFIG_NUM): Adjust value.
	(nios2_init_fpu_configs): Provide register values for new
	-mcustom-fpu-cfg=fph2 option variant.
	* doc/invoke.texi (-mcustom-fpu-cfg=fph2): Document new option
	variant.